298 THE EVOLUTION OF ANIMAL BEHAVIOUR 
of science is, that any natural event he may select for detailed 
study has natural conditions and antecedents. And ib is only 
in such detailed study—taking this or that particular occur- 
~ rence and endeavouring to ascertain what were its related 
antecedents—that advance in the evolutionary interpretation 
of nature can be secured. 
Such advance has been secured by the labours of those 
physiologists who have established by careful experiment the 
quasi-independent action of subsidiary nerve-centres as con- 
stituents of the nervous system as a whole. In such animals 
as the crayfish and the lobster the central nervous system 
consists of a chain of “ ganglia,” or nerve-knots, which are 
connected together by nerve-strands. If these strands be cut 
between the thorax, which carries the walking limbs, and the 
abdomen or hinder portion of the body, the nerve-connection 
between these parts is severed. If the forepart be irritated 
through its sense-organs, the limbs of that part will respond ; 
but, whereas an unmutilated crayfish, subjected to such irrita- 
tion, would give a vigorous flap of the tail, this does not take 
place in the crippled animal.* Still, if the abdomen be 
irritated, it will respond by a strong and swift contraction. 
The two portions of the body are each capable of acting 
independently with well co-ordinated movements, but no longer 
of working together with unity of purpose. In the hinder 
portion the abdominal limbs, or swimmerets, all swing back- 
wards and forwards simultaneously with rhythmic strokes ; 
they act in concert. Sever now the connections between their 
ganglia, and each pair of limbs will continue to swing 
rhythmically but not with concerted rhythm. We have isolated 
a number of quasi-independent centres, and rendered them 
really independent. Hach is concerned with its own proper co- 
ordination, but can no longer combine with others in a wider 
co-ordination. Mr. Hydef has shown that in the king-crab, 
* See Huxley’s book on ‘The Crayfish,” in the International Science 
Series, p. 108. 
+ Journal of Morphology, vol. ix. Quoted by Professor C. S. 
Sherrington in The Marshall Hall Address, “On the Spinal Animal ” 
(reprinted from Medico-Chirurgical Transactions, vol. 82), p. 4. 
